In recent years, it has become common to transmit not only audio data but also large-volume data such as still image data and moving image data in cellular mobile communication systems, in response to spread of multimedia information. Active studies associated with techniques for achieving a high transmission rate in a high-frequency radio band has been conducted to achieve large-volume data transmission.
When a high frequency radio band is utilized, however, attenuation increases as the transmission distance increases, although a higher transmission rate can be expected within a short range. Accordingly, the coverage area of a radio communication base station apparatus (hereinafter, abbreviated as “base station”) decreases when a mobile communication system using a high frequency radio band is actually put into operation. Thus, more base stations need to be installed in this case. The installation of base stations involves reasonable costs, however. For this reason, there has been a high demand for a technique that provides a communication service using a high-frequency radio band, while limiting an increase in the number of base stations.
In order to meet such a demand, studies have been carried out on a relay technique in which a radio communication relay station apparatus (hereinafter, abbreviated as “relay station”) is installed between a base station and a radio communication mobile station apparatus (hereinafter, abbreviated as “mobile station”) to perform communication between the base station and mobile station via the relay station for the purpose of increasing the coverage area of each base station. The use of relay technique allows a mobile station not capable of directly communicating with a base station to communicate with the base station via a relay station.
An LTE-A (long-term evolution advanced) system for which the introduction of the relay technique described above has been studied is required to maintain compatibility with LTE (long term evolution) in terms of a smooth transition from and coexistence with LTE. For this reason, mutual compatibility with LTE is required for the relay technique as well.
FIG. 1 illustrates example frames in which control signals and data are assigned in the LTE system and the LTE-A system.
In the LTE system, DL (downlink) control signals from a base station to a mobile station are transmitted through a DL control channel, such as PDCCH (physical downlink control channel). In LTE, DL grant (also called DL assignment) indicating DL data assignment and UL (uplink) grant indicating UL data assignment are transmitted through PDCCH. DL grant reports that a resource in the subframe in which the DL grant is transmitted has been allocated to the mobile station. Meanwhile, in an FDD system, UL grant reports that a resource in the fourth subframe after the subframe in which the UL grant is transmitted has been allocated to the mobile station. In a TDD system, a UL grant reports that the resource in a subframe transmitted after four or more subframes from the subframe in which the UL grant is transmitted has been allocated to the mobile station. In the TDD system, the subframe to be assigned to the mobile station, or the number of subframes before the assigned subframe in which the UL grant is transmitted is determined in accordance with the time-division pattern of the UL and DL (hereinafter referred to as “UL/DL configuration pattern”). Regardless of the UL/DL configuration pattern, the UL subframe is at least the fourth subframe after the subframe in which the UL grant is transmitted or is a subframe after the fourth subframe.
In the LTE-A system, relay stations, in addition to base stations, also transmit control signals to mobile stations in PDCCH regions in the top parts of subframes. With reference to a relay station, DL control signals have to be transmitted to a mobile station. Thus, the relay station switches the processing to reception processing after transmitting the control signals to the mobile station to prepare for receiving signals transmitted from the base station. The base station, however, transmits DL control signals to the relay station at the time the relay station transmits the DL control signals to the mobile station. The relay station therefore cannot receive the DL control signals transmitted from the base station. In order to avoid such inconvenience in the LTE-A, studies have been carried out on providing a region to which downlink control signals for relay stations are mapped (i.e., relay PDCCH (R-PDCCH) region) in a data region as illustrated in FIG. 2 in LTE-A. Similar to the PDCCH, mapping a DL grant and UL grant to the R-PDCCH is studied. In the R-PDCCH, as illustrated in FIG. 1, locating the DL grant in the first slot and the UL grant in the second slot is studied (refer to Non-patent Literature 1). Mapping the DL grant in the first slot reduces a delay in decoding the DL grant, and allows relay stations to prepare for ACK/NACK transmission for DL data (transmitted in the fourth subframes following reception of DL grant in FDD). Each relay station finds the downlink control signals intended for the relay station by performing blind-decoding on downlink control signals transmitted using an R-PDCCH region from a base station within a resource region indicated using higher layer signaling from the base station (i.e., search space).
As described above, the base station notifies the relay station of the search space corresponding to the R-PDCCH by higher layer signaling. Notification of the search space corresponding to the R-PDCCH may be performed in two different ways: (1) notification using a PRB (physical resource block) pair as a single unit; or (2) notification using an RBG (resource block group) as a single unit. The term, “PRB (physical resource block) pair” refers to a set of PRBs in the first and second slots, whereas the term, “PRB” refers to an individual PRB in either the first or second slot. Hereinafter, a PRB pair may simply be referred to as “PRB.” A resource block group (RBG) is a unit used when a plurality of PRBs are scheduled as a group. The size of an RBG is determined on the basis of the bandwidth of the communication system.
R-PDCCH has four aggregation levels, i.e., levels 1, 2, 4, and 8 (for example, refer to Non-patent Literature (hereinafter, abbreviated as “NPL”) 1). Levels 1, 2, 4, and 8 respectively have six, six, two, and two mapping candidate positions. The term “mapping candidate position” refers to a candidate region to which control signals are to be mapped. When a single terminal is set with one aggregation level, control signals are actually mapped to one of the plurality of mapping candidate positions of the aggregation level. FIG. 2 illustrates example search spaces corresponding to R-PDCCH. The ovals represent search spaces at various aggregation levels. The plurality of mapping candidate positions in the search spaces at the different aggregation levels are continuous on VRBs (virtual resource blocks). The mapping candidate positions in the VRBs are mapped to PRBs (physical resource blocks) through higher layer signaling.
Given the introduction of various apparatuses as radio communication terminals in the future M2M (machine to machine) communication, for example, there is a concern for a shortage of resources in the mapping region for PDCCH (i.e., “PDCCH region”) due to an increase in the number of terminals. If PDCCH cannot be mapped due to such a resource shortage, the DL data cannot be scheduled for the terminals. Thus, the resource region for mapping DL data (i.e., “PDSCH (physical downlink shared channel) region”) cannot be used even if there is an available region, possibly causing a decrease in the system throughput. Studies have been carried out to solve such resource shortage through mapping control signals for terminals served by a base station in a data region to which R-PDCCH is mapped. Then, the resource region including the data region to which the control signal for the terminals served by such a base station is mapped and the data region to which R-PDCCH described above is mapped are referred to an enhanced PDCCH (E-PDCCH) region, a new-PDCCH (N-PDCCH) region, an X-PDCCH region, or the like. Mapping a control signal in a data region in such a manner enables transmission power control for a control signal transmitted to a terminal near a cell edge or interference control for interference to another cell by a control signal to be transmitted or interference to the cell from another cell.
In addition, there is a likelihood that the search space that corresponds to E-PDCCH will be the resource region to which the control signal that is transmitted from the base station to the terminals is mapped. Moreover, the search space that corresponds to E-PDCCH is set for the individual terminals. One part of a design of R-PDCCH also can be used for a design of E-PDCCH, or the design of E-PDCCH can be set to be a design that is entirely different from the design of R-PDCCH. In fact, studies have been also conducted on setting of the design of E-PDCCH and the design of R-PDCCH to be different from each other.
As described above, in the R-PDCCH region, the DL grant is mapped to the first slot, and the UL grant is mapped to the second slot. That is, the resource to which the DL grant is mapped is separated from the resource to which the UL grant is mapped in the time domain. On the other hand, studies have been also conducted on the division of the resource to which the DL grant is mapped and the resource to which the UL grant is mapped in the frequency domain (that is, a subcarrier or a PRB pair) as illustrated in FIG. 3.